Information concerning cardiac and autonomic functions is critical for heart diagnostics and diagnostics of autonomous nervous system, as well as for other medical diagnostics. Accurate monitoring of heart rate and heart rate variability in both time and frequency domains is essential for measuring cardiovascular and autonomic regulatory functions, mental and emotional loads, prediction of dangerous health conditions such as approaching heart attack, stroke and for assessment of health in general.
The arterial blood pressure wave is created during the ventricular systole, and propagates along the arterial tree pumping the blood in the circulatory system. Propagating along the arterial network, the pressure wave causes the distention of arterial walls which can be palpated as an arterial pulse from the major superficial arteries of the body. Doppler ultrasound, arterial tonometry and oximetry are traditional approaches to track the hemodynamics changes in the arteries. Arterial pulse, in most cases, can provide an accurate measure of heart rate and its variability, and can be measured at any point on the body near major superficial arteries. Superficial arteries are found, for example, near the wrist (radial artery), neck (carotid artery), ear (temporal artery), elbow (brachial artery), and groin (femoral artery).
Some devices that measure heart rate, heart rate variability and heart functions use electrodes, or clips, that are placed in contact with the skin. An electrocardiogram (ECG or EKG), which is produced by an electrocardiograph, is commonly used in clinical situations, including critical care, to show heart rate and its variability. The function produced over time, or in the frequency domain, can reveal information that can be useful to medical practitioners when diagnosing diseases. However, placement of the electrodes with the required body contact can be inconvenient, and can slow the acquisition of important data. The use of contact devices, though non-contact, for monitoring the heart condition is undesirable in prolonged monitoring and studies. Moreover, in some applications like magnetic resonance imaging (in which exact event of heart contraction needs to be known to remove the motion artifact on an MRI image due to the heart beat by itself) the use of electrodes disturbs the measurements of the heart rate waveforms because of the strong electromagnetic field.
Various methods have been proposed to access heart performance by measuring blood circulation, such as methods using Doppler Ultrasound, Radar, Laser Vibrometer, Oximetry and Arterial Tonometry. The Doppler, Radar and Laser devices have the disadvantage of relying on active sensors, that is, energy is directed at the body while obtaining the measurements, which energy may have harmful side effects on the health of the individual. Focusing the energy on a proper body parts could also be an issue. Arterial Tonometry and oximetry are contact measurement methods, which, like other contact measurement methods, are somewhat intrusive, require significant subject cooperation and are not favorable in a prolonged continuous monitoring or surveillance application.
Others have proposed to address these problems with non-contact passive methods of obtaining heart function data through the use of imaging. For example, a research group run by Dr. Ioannis Pavlidis at the University of Houston has proposed an image-based technique in which mid-wave Infra-Red (IR) sensors are used to measure cardiac pulse at a distance, by sensing the tissue temperature profile along the superficial blood vessels. See N. Sun, M. Garbey, A. Merla and I. Pavlidis, “Imaging the Cardiovascular Pulse,” Proceedings of the IEEE Computer Society Conference on Computer Vision and Pattern Recognition, San Diego, Cailf., Jun. 20-25, 2005 (hereinafter “Pavlidis et al.”). In this method, a longitudinal linear branch of a superficial blood vessel is manually selected through a graphic user interface and the heartbeat frequency is recovered in the frequency domain by filtering the unwanted artifact frequency peaks. A two to three minute time lapse is used to estimate the heart beat rate by monitoring the manually preselected region along the vessel (a vein). The output is the estimated pulse during the time lapse. However, this method has some limitations, such that only the dominant heart rate frequency is reported, not the actual heart rate waveform, so the valuable interbeat pulse to pulse information and the heart rate variability (i.e., the beat to beat variations) are not determined. The heart beat never stays the same, even within the stated two to three minute intervals. This is why the heart rate variability comes into play. Pulse to pulse variation is itself an important vital sign for indicating the cardiovascular function, autonomic function and physical functioning level. For example, heart rate recovery, such as measured after the first minute after exercise, is a powerful predictor of overall mortality, independent of workload. In elderly or disabled care, a person could pass away during the two to three minute interval before the warning system would inform the nurse. This makes approaches designed for estimation of time averaged heart rate statistics for such long intervals like two to three minutes like in the Pavlidis et. al. method to be impractical for certain situations. Further, due to the analysis methods used, only heart rates between the range of 40-100 beats per minute (bpm) can be reported in the Pavlidis et. al approach. This limitation of the Pavlidis et al. approach prevents detection of certain problems, such as an arterial blockage, during which the heart rate may suddenly increase up to three hundred beats per minute. Finally, this method also has the disadvantage that it requires a medical expert or other highly trained person to manually select the region on the skin to be measured.
While estimated pulse provides useful information, more data concerning heart function is important for many types of diagnoses. Further, minimal invasiveness and speed of data acquisition are also important. Additionally, it is desirable to minimize operator involvement and possible error.